Hydraulic tensioners are widely used to suppress backlash and vibration due to a changes in tension in timing belts and timing chains which transmit rotation from the crankshaft to one or more camshafts in a vehicle engine. A typical hydraulic tensioner comprises a plunger slidable in, and protruding from, a housing. The plunger and housing cooperate to provide a high pressure oil chamber into which oil is introduced through a ball-type check valve for blocking backflow of oil from the high pressure oil chamber.
FIG. 8 shows a conventional hydraulic tensioner 500. In the tensioner, a hollow, cylindrical, plunger 520 is slidable in, and protrudes from, a hole 511 in a housing 510, the plunger being biased in the protruding direction by a spring 530, which extends into the hole 521 formed in the interior of the plunger. The hole 521 of the plunger and the hole 511 of the housing, together constitute a high pressure oil chamber R, which receives oil under pressure from the engine's oil pump or another suitable source through a check valve unit 540, incorporated into the bottom of hole 511. The check valve unit allows oil to flow into the chamber R, but blocks backflow of oil. A typical hydraulic tensioner of the kind depicted in FIG. 8 is described in detail in U.S. Pat. No. 6,203,461.
The plunger is typically arranged to urge a pivoted guide against a traveling chain or other transmission medium to maintain tension therein.
Although the check valve unit 540, in the conventional hydraulic tensioner 500, can block backflow of oil from the high pressure oil chamber R, air can become mixed with the oil in the high pressure oil chamber. The compressibility of the air allows backlash and vibration of the traveling transmission medium, especially upon engine start-up.
Objects of the invention are to solve the above-mentioned problems, and to provide a deairing-type hydraulic tensioner, having a deairing valve unit which can suppress backlash and vibration due to the presence of air in the tensioner's high pressure oil chamber, and which effectively remove air from the interior of the tensioner housing during engine operation while maintaining a high pressure condition in oil chamber. It is also an object of the invention to provide a deairing valve unit that can be easily incorporated into the tensioner housing.